The electrolytic deposition of zinc onto metallic substrates from alkaline solution for the preparation of zinc coated metallic substrates is widely used to prevent such metallic substrates from corrosion and to impart specific optical and mechanical properties on the resulting end product. A process of such electrolytic deposition typically comprises applying a current density to a metal substrate to be zinc coated while placing said substrate in a zinc plating bath. Due to the applied current, zinc ions dissolved in the zinc plating bath deposit on the metallic substrate surface such that a zinc coating is formed thereon.
In the art, several attempts for improving the electrolytic deposition of zinc onto metallic substrates in alkaline solution have been proposed. For instance, US 2012/0138473 A1 refers to a zinc plating bath additive enabling the rapid formation of a zinc coating having small variations in the thickness depending on the position on the surface of an object to be plated. The zinc plating bath additive contains a water soluble copolymer having, as the structural units, two amine compounds. WO 03/006360 A2 refers to an alkaline zinc-nickel electroplating bath that comprises zinc ions, nickel ions, a primary brightener, which is an N-methylpyridinium compound substituted at the 3-position of the pyridine ring with a carboxylate group or a group which is hydrolyzable to a carboxylate group, and a secondary brightener, which is an aliphatic amine. U.S. Pat. No. 3,886,054 A refers to non-cyanide, alkaline electroplating baths for bright zinc plating containing quarternized polymeric condensates of alkylene polyamines and 1,3-dihalo-2-propanol as grain refiners preferably in admixture with aldehyde type brighteners and mercapto substituted heterocyclic compounds capable of producing bright, fine grained deposits over a broad current density range. US 2005/133376 A1 refers to an aqueous zinc-nickel electroplating bath, including water; nickel ion; zinc ion; at least one complexing agent; and at least one non-ionogenic, surface active polyoxyalkylene compound, wherein the bath has an alkaline pH. However, the preparation of zinc coated metallic substrates by the electrolytic deposition of zinc or zinc alloy onto the substrate is challenging. For instance, during the electrolytic deposition of zinc or a zinc alloy onto a metallic substrate, hydrogen is generated which tends to adhere on the coating surface as small gas bubbles resulting in a zinc or zinc alloy coating formed on the metallic substrate having worsened optical appearances. Such worsened optical appearances are typically visible on the surface in the form of stripes. In addition thereto, such formation of bubbles also decreases the adhesion of the zinc coating on the metallic substrate which may be detectable as small blisters on the surface and thus also decreased mechanical properties are obtained. Thus, the addition of surfactants into the plating bath would be desirable in order to support the formation of an even coating on the metallic substrate and thus to improve the optical appearance of the zinc or zinc alloy coated metallic substrate surface. In this regard, it is to be noted that surfactants considered as being suitable in zinc plating processes should be soluble in the plating bath. However, such water-soluble surfactants also tend to stabilize foams generated during the deposition process which may then interfere with the deposition of zinc or zinc alloy on the metallic substrate such that an uneven coating is formed thereon resulting again in optically deteriorated appearances. In contrast thereto, surfactants which are known as being sufficient as regards the non-stabilization of foams are typically insoluble in the aqueous zinc plating bath and are thus considered unsuitable in such baths.
Thus, there is a need in the art for providing a process which avoids the foregoing disadvantages and especially allows for the preparation of a zinc or zinc alloy coated metallic substrate imparting very well optical characteristics to a resulting end product while its mechanical properties are kept on a high level or even improved. In particular, it is desirable to provide a process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate which is well balanced with regard to optical appearances resulting from the non-formation of foam and bubbles in the plating bath on the one hand and the adhesion of the zinc or zinc alloy coating on the metallic substrate on the other hand.
Accordingly, it is an object of the present invention to provide a process for the electrolytic deposition of a zinc or zinc alloy coating on a metallic substrate. Furthermore, it is an object of the present invention to provide a process in which a zinc or zinc alloy coating is formed on the metallic substrate having an even thickness. It is an even further object of the present invention to provide a process in which the optical appearance of the resulting zinc or zinc alloy coating formed on the metallic substrate is improved. Another object of the present invention is to provide a process in which the mechanical properties of the resulting zinc or zinc alloy coating formed on the metallic substrate are kept on a high level or are even improved. A still further object of the present invention is to provide a process in which a good wetting of the metallic substrate surface is obtained such that an improved release of gas bubbles is caused improving the optical appearance of the resulting zinc or zinc alloy coated metallic substrate. A further object of the present invention is to provide a process in which the obtained zinc or zinc alloy coated metallic substrate is the result of well-balanced properties with regard to the wetting behavior as well as the adhesion of the zinc or zinc alloy coating on the metallic substrate. Further objects can be gathered from the following description of the invention.